1. Field of the Invention
The present invention relates to a battery device including a secondary battery and a charge and discharge control circuit configured to detect a voltage and abnormality of the secondary battery to control charge and discharge of the secondary battery, and more particularly, to a charge and discharge control circuit and a battery device that are capable of preventing a battery from entering an abnormal state or preventing an excessive current from flowing through a battery or an apparatus connected to the battery.
2. Description of the Related Art
FIG. 4 is a circuit diagram illustrating a related-art battery device including a charge and discharge control circuit. The related-art battery device including the charge and discharge control circuit includes a secondary battery 11, an N-channel discharge control field effect transistor 12, an N-channel charge control field effect transistor 13, a charge and discharge control circuit 14, resistors 22 and 31, a capacitor 32, and external terminals 20 and 21. The charge and discharge control circuit 14 includes a control circuit 15, an overcurrent detecting circuit 530, a short-circuit detecting circuit 540, an overcurrent detecting terminal 19, a charge control signal output terminal 41, a discharge control signal output terminal 42, a DS terminal 45, a positive electrode power supply terminal 44, and a negative electrode power supply terminal 43. The overcurrent detecting circuit 530 includes a comparator circuit 101 and a reference voltage circuit 531. The short-circuit detecting circuit 540 includes a comparator circuit 102 and a reference voltage circuit 541.
The control circuit 15 includes resistors 504, 505, 506, 507, 518, and 528, reference voltage circuits 509 and 515, comparator circuits 501, 508, and 513, an oscillator circuit 502, a counter circuit 503, a logic circuit 510, a level shift circuit 511, a delay circuit 512, a logic circuit 520, and NMOS transistors 517 and 519.
Next, an operation of the related-art battery device including the charge and discharge control circuit is described. When a load is connected between the external terminals 20 and 21 and a current flows, a potential difference is generated between a negative electrode of the secondary battery 11 and the external terminal 21. This potential difference is determined based on a current amount I1 flowing between the external terminals 20 and 21, a resistance value R12 of the N-channel discharge control field effect transistor 12, and a resistance value R13 of the N-channel charge control field effect transistor 13, and is represented by I1×(R12+R13). A voltage of the overcurrent detecting terminal 19 is equal to a voltage of the external terminal 21. The comparator circuit 101 compares a voltage of the reference voltage circuit 531 with the voltage of the overcurrent detecting terminal 19. When the voltage of the overcurrent detecting terminal 19 is higher, the N-channel discharge control field effect transistor 12 is turned off for overcurrent protection. A setting value of an overcurrent detection current value is represented by IDOP, a voltage of the reference voltage circuit 531 is represented by V531, a resistance value of the N-channel discharge control field effect transistor 12 is represented by R12, and a resistance value of the N-channel charge control field effect transistor 13 is represented by R13. A voltage of the external terminal 21 as a threshold voltage for the comparator circuit 101 to output a detection signal is V531. At this time, the current flowing between the external terminals 20 and 21 is obtained by dividing the voltage of the external terminal 21 by the sum of the resistance values of the N-channel discharge control field effect transistor 12 and the N-channel charge control field effect transistor 13, and is represented by IDOP=V531/(R12+R13).
A voltage of the overcurrent detecting terminal of the charge and discharge control circuit as a threshold voltage for the comparator circuit 101 to output a detection signal is referred to as “overcurrent detection voltage”. A voltage of the overcurrent detecting terminal of the charge and discharge control circuit as a threshold voltage for the comparator circuit 102 to output a detection signal is referred to as “short-circuit detection voltage”.
However, in the related art, the overcurrent detection voltage and the short-circuit detection voltage of the charge and discharge control circuit have constant values even when the secondary battery voltage or temperature changes, but the resistance value of the N-channel charge and discharge control field effect transistor changes along with a change in the secondary battery voltage or temperature, resulting in fluctuations in an overcurrent detection current value and a short-circuit detection current value. Accordingly, there is a problem in that the overcurrent detection current value and the short-circuit detection current value are low in accuracy to reduce the safety of the battery device. Further, there is another problem in that current consumption is high because two reference voltage circuits are used for the overcurrent detecting circuit and the short-circuit detecting circuit.